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The aim of this paper is to propose an efficient numerical scheme to deal with heat transfer problems in pipe flow with a large length/diameter ratio. The generalized finite difference method (GFDM) is combined with a space stepping algorithm (SSA) for the solution. The SSA divides the solution domain into a number of overlapping sections in the length direction of the pipe. Due to the large Peclet number, the heat transport process is dominated by advection, which allows an approximate boundary condition to be applied at the downstream cross section. The problem is then solved section by section. Using the uniform distributed points, the solution matrix for each section does not change, and only the right hand side needs to be refreshed for the changing boundary conditions. This leads to a highly efficient scheme for problems with a large pipe length. To show the accuracy of the numerical scheme, a problem with available analytical solution is studied. Then the method is applied to problems of single pipe with different pipe wall temperature and flow Reynolds number, and concentric annular pipe. Numerical results confirm the stability and efficiency of the GFDM-SSA. The method can be applied to many real world transient heat transfer problems in which the pipe is heated or cooled along its length with arbitrary wall temperature for heat exchange and other purposes. [ABSTRACT FROM AUTHOR]

This research paper focuses on CFD analysis for comparative evaluation of different hybrid nanofluids (NFs): MWCNT-Al 2 O 3 /H 2 O, MWCNT-MgO/H 2 O, Al 2 O 3 -CuO/H 2 O performance as working fluid (WF) inside parabolic trough solar collector (PTSC). The performance of the collector is evaluated using different performance indicators, including Nusselt number (Nu), friction factor (f), and Performance Evaluation Criterion (PEC) for different Reynolds number (Re) (10000-50000). Among the investigated hybrid nanoparticles (NPs), MWCNT-Al 2 O 3 emerges as the most promising choice for significantly enhancing the system's thermal performance with the highest PEC value of 1.9. This outcome signifies the superior efficiency of MWCNT-Al₂O₃/H₂O hybrid NF in enhancing convective heat transfer, thus underlining the paramount significance of adopting them in thermal systems operating within the specified Re range. [ABSTRACT FROM AUTHOR]

This paper is the second of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. This paper focuses on the effects of different flow directions and two tube wall thicknesses. Additionally, this paper also provides an analysis on the chilldown thermal efficiency of the quenching process. Three flow directions with four different inner surface coating modifications, and three tube wall thicknesses were examined. The experimental data covers the Reynolds numbers ranging from 3500 to 140,000. The chilldown efficiency, along with chilldown time and LN 2 (liquid nitrogen) mass consumption were analyzed to assess the overall performance of the LN 2 line chilldown process. For thin tube wall cases, the chilldown efficiencies cover a range between 3% and 41%, and the maximum chilldown efficiency value is found for the tube with 3 L coating at Re = 5278 in the vertical up flow direction. For thick wall tube cases, the efficiencies cover a range between 4% and 52%, and the maximum chilldown efficiency value was found for the tube with 4 L coating at Re = 5308 in the vertical up flow direction. [ABSTRACT FROM AUTHOR]

Hyperthermia is one of the non-invasive therapy of an Abdominal Aortic Aneurysm (AAA), which is achieved by applying a heat source upon the AAA without surgical operation. This research paper considers laminar flow and heat transfer in a heated abdominal aortic aneurysm using an isothermal boundary condition. Heat is added to explicate the thermal treatment of a diseased artery. The blood is assumed as a non-Newtonian fluid based on the shear-thinning Carreau model. Two unequal aneurysms are assumed in the lower wall to simulate bulges or a disordered artery. Flexible wall segments are assumed in the upper wall and opposing to each aneurysm. The transient momentum and energy equations are solved based on the fluid–structure interaction (FSI) using the Arbitrary-Lagrangian–Eulerian (ALE) method. It is found that the shear stress is much higher for a higher index of the power-law fluid governing the blood viscosity and hence, it is strictly recommended to reduce the viscous nature of the blood in diseased vessels. It is found also that the thermal energy can be greatly transported across the blood at a higher Reynolds number, this means that the hyperthermia therapy becomes effective when blood flows violently through the aortic. [ABSTRACT FROM AUTHOR]

In this paper, the gas turbine blades middle double swirl cooling structure under diverse number of nozzles N are constructed to explore the heat transfer and flow behavior. The k-ω turbulence model are applied and the inlet coolant Reynolds number is 12500. The N vary by 4 to 11 and two cases are investigated. For case 1, the cross-sectional area of each nozzle is 6 mm2 for all structures. For case 2, the cross-sectional area of total nozzles stays the constant at 42mm2 when the N is varied. Results show that, in case 1, when the N increases, the area of the high heat exchange zone decreases but the distribution is more uniform. Moreover, the thermal performance factor ξ gets larger, but the average Nusselt number Nu a decreases significantly. In case 2, when the N increases, the ratio of inclination to nozzle length increases for the jet near the outlet, Nu a is almost constant but the heat transfer uniformity is improved. The best ξ is obtained when the N is 8. For case 1, as the N is increased, the Nu a decreases significantly. Therefore, heat transfer and ξ should be considered by case 2 to select the optimum N. • Middle double swirl cooling models with different nozzle numbers are established. • Effects of nozzle number on flow and heat transfer characteristics are explored by two cases. • When the nozzle number is changed, the cross-sectional area of total nozzles in case 1 is changed, but it unchanged in case 2. • The results show that case 2 is more appropriate and the optimum nozzle numbers should be selected by the way of case 2. [ABSTRACT FROM AUTHOR]

This paper is the first of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. The experimental data covers a wide range of flow mass fluxes (43 to 1077 kg/m2s). The experiments were conducted using test tubes with four different coating thicknesses to investigate the effects of various Teflon coating thicknesses. An analysis of the chilldown time and LN 2 mass consumption data are presented to evaluate the effects of the flow mass flux, axial distance from the tube inlet, and coating layer thickness. The chilldown time decreases with increasing flow Re. However, the chilldown mass consumption increases with increasing flow Re. In general, the larger the flow Reynolds number, the lower the percentage reduction in chilldown time is observed. For the percentage reduction in the chilldown time by the coating, the minimum is 27.8% for the tube with 7-layer coating at flow of Re = 100,000 and the maximum is 61.3% for the tube with 14-layer coating at flow of Re = 10,000. The percentage reduction in mass consumption due to coating falls in the range between 35.3% and 54.1%. [ABSTRACT FROM AUTHOR]

This paper explores the impact of hybridisation and geometric variations in micro heat exchangers featuring cylindrical fins. The primary aim of the study is to maximise the thermal conductance. Solid, half-hollow and hollow circular fins are modeled and mounted on a heat sink with a rectangular flow channel. A high-density heat flux of magnitude 500 W / cm 2 is dissipated on the bottom surface by a microelectronic device. Single-phase water of Reynolds number range of between 400 and 500 is supplied through the inlet of the heat sink to remove the heat at the bottom and in the inner walls of the hollows, while cool air stream of Reynolds number range 10 ≤ Re a ≤ 14 is allowed to convectively flow over the vertical micro cylindrical fins to take away extra heat deposited on the fins. The computation domain is descretised and computational fluid dynamic code applied to solve mathematical equations. The numerical results show that the thermal conductance in the half-hollow fins is 16.4% greater than in the solid fins, and 0.14% higher than the hollow fins in parallel flow, while in counter flow the half-hollow fins increase above the solid fins by 5% and by 0.5% higher than hollow fins. The integrated micro heat sink with half-hollow is superior in both parallel and counter flow. Experimental data is used to validate the numerical prediction. [Display omitted] • A groundbreaking method for cooling micro heat exchangers. • Dual cooling technique in integrated heat sink. • The application of cylindrical fins in an integrated heat sink. • A novel approach to enhance heat transfer efficiency in heat sinks. • A cutting-edge innovation in heat sinks with a hybrid configuration. [ABSTRACT FROM AUTHOR]

Rising temperature of gas turbine inlet brings great challenge for blade temperature tolerance, thus high effective cooling structure for turbine blade is vital. In this paper, aerothermodynamic features including heat transfer and pressure drop of a Coriolis-applied rotating smooth U channel are studied in detail. Both experiment and numerical methods are utilized in the study. A rotating cooling structure experiment rig system has been established and validated. The experiments were conducted in the rotating cooling structure experiment rig. Besides, RANS was chosen in the simulation. In the experiment and numerical studies, Reynolds number is constant at 16000 and Rotation number ranges from 0 to 0.024. The results indicate that Coriolis force plays a dominant and positive role on heat transfer ability on trailing wall, and that bend outlet impingement effect is the primary factor compared to Coriolis force for the Nusselt number of smooth leading surface. Besides, Coriolis force at bend region probably suppresses the formation of K–H vortices, thus boosts the flow stability, reduce pressure loss of channel and weakens second heat transfer peak of impingement at the bend outlet. What's more, the total performance of the Coriolis-applied smooth U channel at Ro of 0.006 and 0.024 are 2.18 % and 8.51 % higher than the channel at Ro of 0, which means that rotation is good for the Coriolis-applied smooth U channel and that the channel has superior performance and is encouraging in future rotating blade cooling application. • A Coriolis-applied smooth U channel is proposed and investigated. • Detail heat transfer experiments on the channel were implemented. • Simulations were conducted to unveil the heat transfer and pressure drop mechanisms. • Rotation enhances overall heat transfer of the Coriolis-applied smooth U channel. • Rotation brings pressure loss of the Coriolis-applied smooth U channel decreased. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of heat transfer enhancement and pressure drop in a Coiled Flow Reverser (CFR) with a twisted tape insert. Both laminar (10 < Re < 2000) and turbulent (12,000 < Re < 18,000) regimes have been studied using both experimental and numerical methods. The effects of twist ratio of twisted tape, inlet Reynolds number, and perforating the twisted tapes have been investigated. The study found that the best performance in the laminar regime is achieved with a twisted tape with a twist ratio of 15.73 and no holes on it at a Reynolds number of 2000. In the turbulent regime, the same twisted tape performs the best at a Reynolds number of 17,000. The highest Thermal Performance Ratio (TPR) in laminar regime is 1.55 and in turbulent regime is 0.99. In comparison to the straight pipe, all types of CFRs show better performance in heat transfer enhancement. Also, as the Reynolds number increases, the friction coefficient decreases in all configurations. The study reveals that the twisted tape insert enhances heat transfer performance while increasing pressure drop. This study provides valuable insights into the design and optimization of heat exchangers with CFR. The findings can be used to improve the efficiency and performance of various industrial processes, such as refrigeration systems, air conditioning, and power plants. • Fluid flow and heat transfer in a coiled flow reverser (CFR) were studied. • Effects of tube insert on the heat transfer enhancement of CFR were investigated. • Effects of perforation of tube insert on heat transfer enhancement of CFR were examined. • The effects of tube insert at laminar flow are more than turbulent flow. • Perforation has no significant effects on the thermal performance ratio. [ABSTRACT FROM AUTHOR]

With the development of science and technology, the heat dissipation problem of high-density electronic products has gradually emerged in engineering practice. In this paper, the enhanced heat dissipation process of fluid induced oscillation through flexible membrane is proposed, which belongs to the multi-field synergistic bidirectional coupling effect of thermal-fluid-solid coupling. Through the comparative numerical analysis of the flow induced oscillation of the flexible membrane and the fixed rigid fin in the flow channel, the mechanism of the fluid induced oscillation of the flexible polyethylene membrane is revealed. Based on the two-way fluid-solid coupling method, a numerical model for the heat transfer process of polyethylene membrane with thickness of 0.02 mm in the fluid channel was established. The temperature field, Nu (Nusselt number) and PEC (Performance comparison index) number of the rigid fin and polyethylene membrane installed in the channel under different Reynolds numbers were calculated respectively. The ratio of the length of the membrane to the height of the channel was adjusted for multiple groups of calculations to verify the positive effect of the polyethylene membrane installed in the channel on the heat exchange effect. The results show that in the fully developed area of heat transfer, the channel with polyethylene membrane is better than that with rigid fins in terms of turbulence and vortex shedding; at Re = 4400, the comprehensive enhanced heat transfer factor of heat exchange is the largest, up to 1.27; as the ratio of the height of the new flexible membrane to the height of the channel is 1, the heat transfer enhancement effect is best. In the next step, the correlation theory between oscillation characterization parameters and heat transfer enhancement effect under pulsating fluid excitation will be investigated. [ABSTRACT FROM AUTHOR]

• The flow and heat transfer characteristics of single and multiple-synthetic jet impingement cooling is presented. • The equivalent Reynolds and Nusselt numbers were determined for the purpose of describing multiple orifice configuration. • The merging jet process is presented and the merging point is determined. In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p / d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions. [ABSTRACT FROM AUTHOR]

In order to enhance the heat transfer performance of the battery thermal management system, to obtain a more uniform temperature distribution and to achieve the lightweight design requirements, a new liquid cooling plate design approach with primary and secondary combined fins synergistic phase change materials and nanofluid is proposed in this paper. Based on the traditional square fins, 10 new improved fin design options are proposed in this paper. Firstly, the thermal and flow characteristics of the 10 new improved fins are compared and analyzed, and the optimization is carried out. Next, the optimal model is obtained by considering the primary fin size and introducing the secondary fins of the partitioned combination for optimization. Compared to the traditional square fins, the average temperature is reduced by 0.288 °C and the pressure drop is reduced by 1.287 Pa (17.42%). Then, based on the optimal model, the thermal performance of the combined filling method and thickness of PCM is discussed and analyzed for different Reynolds number conditions. The results indicated that the smaller the Reynolds number, the more significant the heat dissipation effect of the filled PCM. The optimal model with PCM filling reduces the mass by 41.4 g (68.86%) compared to the model without PCM, with significant lightweighting effect. Finally, to further improve the thermal performance of the system, different types of nanoparticles with different volume fractions are introduced into the system. Compared to pure water liquid cooling, the Nusselt number can be increased by 9% to 78%, and the heat dissipation effect is significantly improved. • Based on the traditional directional fins, 10 new and improved fins are proposed. • Thermal performance of the system was improved after introduction the secondary fins. • A new liquid cooling plate with PCM synergistic combination fins was proposed. • The mass of the optimal model after filling the PCM was reduced by 41.4 g (68.86%). • Nusselt number of system can be improved by 9% to 78% after introduction nanofluid. [ABSTRACT FROM AUTHOR]

• The heat transfer process of NH 4 Cl fouling was studied experimentally. • The local heat transfer characteristics around tube wall was analyzed. • The effects of flow rates and temperature on heat transfer process were investigated. • The fouling resistance and average fouling thickness on the HET wall were obtained. The crystallization fouling of ammonium chloride (NH 4 Cl) often occurs on heat exchange tube bundles in petroleum refining equipment, which seriously reduces the heat transfer and operational efficiency of the equipment. In this paper, the heat transfer characteristics of heat exchange tube (HET) with NH 4 Cl crystallization fouling was studied experimentally. The effects of velocity, temperature and mass flow rate of hot and cold fluids on the overall and local heat transfer characteristics were analyzed. The results indicate that the local heat transfer coefficient around the tube wall decreases first and then increases with the increase of the circumferential angle (θ) under clean condition. At the position of 0°-45°, the local heat transfer coefficient is the largest. At 90°-135°, the local heat transfer coefficient is the smallest. Under fouling condition, the local heat transfer coefficient increases with the increase of circumferential angle. In the region of 0°-45°, the local heat transfer coefficient is the smallest. The region with the largest local heat transfer coefficient is 135°-180° The heat transfer coefficient (h), fouling resistance (R f) and average fouling thickness increase with the increase of Reynolds number (Re), temperature of hot fluid (T h) and mass flow rate of cold fluid (q m,c), but decrease with the increase of temperature of cold fluid (T c). [Display omitted] [ABSTRACT FROM AUTHOR]

• The pressure drop through polyurethane foams was studied. • A correlation for friction factor based on fiber diameter and permeability in polyurethane foams was obtained. • A correlation for Nusselt number based on fiber diameter in polyurethane foams was obtained. • Comparison of heat transfer and friction factor in polyurethane foams with metal foams was made. Foam-like materials have a large specific surface area and complex structure that promotes flow turbulence, which makes them useful for heat transfer. In the literature, heat transfer in metal foams (MF) with high thermal conductivity is mainly considered. In the present paper, an experimental study of the hydraulic and heat transfer characteristics of a rectangular channel filled with polyurethane foam (PUF) inserts with a thermal conductivity of 0.2 W⋅m−1⋅K−1 in the air flow was carried out. The open-cell PUF samples had 20 and 80 pores per inch (PPI), 0.97 and 0.98 porosity, 269 µm and 60 µm fiber diameter, respectively. The Reynolds number, based on the fiber diameter ranged from 0.04 to 50. From the results of hydraulic tests, friction coefficients were determined based on the fiber diameter and permeability. The Forchheimer coefficient was 0.198 and 0.318 for 20 and 80 PPI samples, respectively. The permeability was 1.889 10−7 ⋅m2 and 7.535 10−9 ⋅m2 for 20 and 80 PPI samples, respectively. The Nusselt number for both tested PUF samples was correlated with the Reynolds number based on the fiber diameter in a power law with the Reynolds number exponent and constant equal to 0.61 and 0.037 respectively. The intensity of heat transfer in the PUF samples was 7–12 times lower than that in MF. However, a significant heat transfer enhancement is still possible compared to the empty channel up to 7 times. The heat transfer performance of PUF was higher compared to the empty channel, but the thermal performance factor was lower than one. Nevertheless, in practical situations where the mass or cost of the heat exchanger is preferable, PUF can be considered as a heat transfer intensifier. [Display omitted] [ABSTRACT FROM AUTHOR]

• Reduced gravity cryogenic line chilldown data presented. • Investigation is carried out on effect of low thermally conductive coatings and pulse flow. • Comparisons are made between 1-g and reduced gravity for similar conditions. • No appreciable degradation in chilldown efficiency at moderate-to-high Reynolds numbers in reduced gravity relative to terrestrial gravity. Future cryogenic propulsion systems will require efficient methods to chill down transfer lines and propellant tanks prior to transfer of propellant while in space. When chilling down the transfer line, there is an inherent tradeoff between propellant mass consumed and chilldown time to steady state, depending on mission constraints and desires. This paper examines an efficiency parameter based on a simplified energy balance that can be used to compare the performance of different chilldown methods and to determine the optimal chilldown method. The parameter, along with chilldown time and mass, is then applied to recent liquid nitrogen line chilldown experiments conducted in both terrestrial and microgravity to compare efficiencies between numerous continuous and pulse chilldown tests on bare as well as coated tubes across a range of thermodynamic conditions. Despite a well-known reduction in heat transfer in microgravity due to the absence of buoyancy-assisted cooling, results indicate no appreciable degradation in chilldown efficiency at moderate-to-high Reynolds numbers in microgravity relative to terrestrial gravity. [ABSTRACT FROM AUTHOR]

In this paper, the fluid flow behavior and cooling characteristics of a sweeping impingement jet were investigated. Also, the difference between free and confined impingement schemes was investigated. In addition, the conventional approach for the purpose of significantly reducing computational time by using a confined domain with a slip upper wall instead of an unconfined domain was evaluated. A 2D unsteady Reynolds averaged Navier-Stokes (URANS) simulation accompanied with the k-ω SST turbulence model is used in this study. The study has been conducted for a target wall with a constant heat flux of 3000 W/m2, jet-to-wall distance of 4, and a jet Reynolds number of 2500. The results show that the overall average cooling performance of the sweeping jet is better in the confined impingement scheme compared to that of the steady jet, while the steady jet is slightly better in the unconfined sweeping impingement scheme. Using a confined scheme with a slip upper wall does not reveal the complete thermal and flow behaviors, and the wall heat transfer distribution is very different from the unconfined domain. [ABSTRACT FROM AUTHOR]

This paper studied the buoyance-aided mixed convective heat transfer of air-cooling flow in a 5 m long and 1.2mX0.3 m cross-section vertical channel. One of the channel walls was uniformly heated and the rest three walls were unheated and insulated. Heat was transferred from the heated wall by convection and radiation, and heat was also transferred from unheated walls to the air flow by convection originated from their net radiation gain from the heated wall. The air flow rate, the inlet and outlet air flow temperatures, and local surface heat fluxes and temperatures on the heated wall were measured. The Reynolds number of air flow was from 2900 to 221,000 and Richardson number was changed from 0.006 to 90.6. The results showed that for walls with larger emissivity the total convection heat transfer was not deteriorated in mixed convection zone. Therefor a modified heat transfer correlation was developed for the aiding mixed convection in the asymmetrically heated channel considering both mixed convection and radiation. It predicted the experiment results with a deviation less than 18%. [ABSTRACT FROM AUTHOR]

In this paper, thermohydraulic characteristics and enhanced heat transfer mechanism in water–cooling oil heat exchanger with various shapes and attack angles of vortex generators (VGs) and spacing of double VGs are numerically investigated at different Reynolds number (Re) and oil–water mediums. A CFD-aided method, conjugated heat transfer model, is performed to elaborate the correlation between heat transfer and flow characteristics. The quantitative and qualitative results manifest that the longitudinal and secondary vortices, induced by VGs, generate synergistically the tapered and elliptical enhanced heat transfer regions on the heat exchange fins, but the inhibiting effect still exists between both, particularly in high attack angle or oil medium. Elliptical winglet VG with 30° in oil medium with Re = 349 and 15° in water medium with Re = 2726 gains the maximum thermal performance factor, reaching 1.69 and 1.28, respectively, compared without VGs. Furthermore, the appropriate configuration of two core heat transfer regions is a cost-optimal method to obtain in-depth synergistic heat transfer. The spacing of 4 mm shows the best synergistic heat transfer, and the maximum heat flux of its was elevated to 2.2% and 8.6% more than that of single VG in oil and water medium, respectively. [ABSTRACT FROM AUTHOR]

The droplet impacting on a substrate is widely encountered in daily life and industrial applications. In this paper, the impact dynamics and heat transfer are numerically simulated for both single droplet and binary droplets on a hot superhydrophobic substrate. The dimensionless numerical model is built up through the transient 2D axisymmetric model with a volume of fluid (VOF) model. The effect of the Weber number, Reynolds number, size ratio, contact angle on the spreading dynamics and heat transfer is investigated in details. It is found that the spreading factor and contact time increase with the increasing Weber number. Besides, the maximum spreading factor and contact time of binary droplets are larger than those of a single droplet under the same Reynolds and Weber numbers. The transient heat transfer rate of a single droplet is larger than that of binary droplets impingement due to larger spreading surface area-to-volume. The total input heat of a single droplet is generally larger than that of binary droplets except at large Weber numbers, and the equal-sized binary droplets have the larger total input heat than un-equal-sized binary droplets. For binary droplets impact on a hot superhydrophobic surface, the hot substrate can promote the spreading and retard the receding due to thermo-capillary effect, and thus will enhance heat transfer between the droplet and the hot substrate. These findings may be helpful in gaining insights into the dynamics and heat transfer of binary droplets impact on the hot substrate. [Display omitted] • Spreading factor and contact time of binary droplets are higher than single droplet. • Heat transfer of single droplet is higher than binary droplets. • Heat transfer is the best for equal-sized droplet. • Thermal-capillary effect promotes the spreading and retards the receding. [ABSTRACT FROM AUTHOR]

Augmentation of heat transfer without increasing the pressure drop has been a challenge in turbine blade internal cooling. Ribs are commonly used to achieve this goal and it is imperative to keep the pressure loss as low as possible. In this paper, the thermohydraulic performances of a number of perforated ribs are examined and compared. Three geometrical features of the ribs include hole inclination angle (0°, 30°, and 45°), relative hole inlet height (0.2, 0.4, 0.6, and 0.8), and the variation of the cross-sectional area of the circular holes (convergent, straight, and divergent). The ribs are mounted in a rectangular channel (A R = 2:1) and flows with a range of Reynolds numbers (10,000 to 25,000) are examined. A validated SST Gamma-Theta model is employed to evaluate the turbulent heat transfer and pressure loss characteristics. It is found that the permeable rib labeled as Case #212, improves the performance between 4.35 and 6.39% for flows with Reynolds numbers between 10000 and 25000. Among the investigated configuration parameters, the hole inclination has the highest effect on the thermal performance enhancement. • Study on heat transfer and pressure drop characteristics of perforated ribs. • Role of each geometric parameter of the perforated rib is determined. • Flow structure is investigated to obtain fundamental reasons for performances. • Best performance by perforated ribs for Reynolds number from 10,000 to 25,000. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model is proposed to study the transition to three-dimensionality and the evolution of vortices in the wake of a blunt-headed cylinder considering air as the working fluid in the Reynolds number (Re) range 130–2000. An experimental setup is developed to validate the numerical model on the basis of heat transfer characteristics. Based on symmetry and time dependence characteristics of flow, it is categorized as dynamically steady symmetric flow (50 ≤ Re ≤ 200), dynamically steady unsymmetric flow (220 ≤ Re ≤ 270) and unsteady flow (Re ≥ 280). The vortex dislocation phenomenon showing intermittent low frequency pulsations during the transition to three-dimensionality is studied. Aerodynamic characteristics are studied by computing force coefficients and Strouhal number (St). Brief overview of heat transfer characteristics is given. Dynamic mode decomposition (DMD) analysis is carried out to determine quantitatively the critical Re at which secondary instability occurs and hence clear demarcation of the Re regime is obtained for the validity of two-dimensional analysis of blunt headed cylinder. [ABSTRACT FROM AUTHOR]

• Kagome truss structure of water-drop-shaped rods is proposed. • Flow and heat transfer characteristics of the composite structures are conducted. • The thermal performance of the new structure is better than the reference structure. • The best thermal performances of the composite structures with cross-flow are found. Jet array impingement providing high heat transfer are widely used in heat exchangers. However, the target plate is easily damaged impinged by high-speed air flow constantly. To solve this issue, Kagome truss structure is combined with jet array impingement to improve cooling performance in this paper. The flow and heat transfer characteristics of this composite structure in a heat exchanger unit with/without cross-flow were experimentally studied. Additionally, Kagome truss structures of circular and water-drop-shaped rods were used respectively in jet space and the results showed that the latter was beyond 7% higher than the former for the comprehensive thermal performance. When cross-flow was not added, the comprehensive thermal performance factor of the latter reached to a maximum of 1.656 at the Reynolds number of 5000. When cross-flow was added, the comprehensive thermal performance got worse at the cross-flow-to-jet velocity ratio of 1.0, while the best comprehensive thermal performances were found at the cross-flow-to-jet velocity ratio of 0.6 and at the mass flow rate proportion of cross-flow of 0.5, respectively. These results can provide a basic experimental data as a reference for designing new structures in advanced heat exchanger. [ABSTRACT FROM AUTHOR]

In this paper, the middle double swirl cooling (M-DSC) models for diverse nozzle aspect ratios (C ar) are established for flow and heat transfer behavior research. The k-ω turbulence model and the 3D viscous steady RANS equations are utilized for numerical calculation. The finite volume method and coupled implicit solver are chosen for simulation calculation. Influences of C ar values, namely 0.375, 0.67, 1.5, 2.67, 4.17 and 6 on the flow characteristics and thermal performance for the M-DSC are investigated when keeping vortex nozzle inlet area constant. Moreover, the cooling performance for diverse inlet Reynolds numbers 12,500, 22,500, 32,500 and 42,500 is studied with the model owning the optimal C ar. Results show that the velocity of the crossflow will increase along in the outlet direction. As C ar decreases, the velocity where the jet coolant scours the target will decay more significantly toward the outlet. There are four typical shapes of high heat transfer regions on the target. The uniformity of heat transfer will increase with the growth of C ar. When the C ar is 4.17, the thermal performance factor is the highest. The flow and heat transfer regulations are almost constant when the inlet Reynolds number is altered. • Middle double swirl cooling models with different nozzle aspect ratios are established. • Effects of different nozzle aspect ratios and inlet Reynolds numbers on flow and heat transfer characteristics are explored. • Optimal nozzle aspect ratio with the best cooling performance is obtained. [ABSTRACT FROM AUTHOR]

Nowadays, utilization of micro/mini channel heat sinks (MCHS) has become a popular solution for cooling of electronic systems and maintaining their operating temperature in a desired range. Increasing the contact surface area between fluid and solid zones by geometrical modifications is a promising solution that significantly affects thermal performance and heat transfer in these systems. In this paper, considering thermohydraulic performance, the effect of creating grooves on the middle-placed barriers of a minichannel heat sink is investigated to access a criterion for favorable temperature performance. Evaluating different geometrical parameters of barriers and grooves including cross section, depth and position of horizontal groove as well as vertical groove distance, simultaneous employment of optimal horizontal and vertical groove in the channel at different Reynolds numbers is studied. Outputs including channel base temperature, friction factor ratio and total thermal resistance are studied to evaluate the performance of the proposed minichannel. Results indicate that vortex formation adjacent to solid walls considerably enhances the heat transfer and variation of each geometrical parameters offers a performance enhancement. Moreover, the best thermal performance is obtained by simultaneous employment of the horizontal rectangular groove (x = 0.45 mm, y = 0.2 mm and h g = 0.6 mm) and the vertical groove (d g = 0.4 mm). In the optimal case, the channel base temperature and the total thermal resistance reduces by 11.88 K and 27%, respectively. [ABSTRACT FROM AUTHOR]

The present paper compares measured and calculated heat transfer parameters for a small-size slot supersonic nozzle. Heat transfer coefficient and temperature recovery factor were examined under conditions of partial laminarization of boundary layer caused by moderate flow acceleration and low Reynolds numbers. The transient heat transfer method was implemented for estimation of these values for the nozzle subsonic and supersonic parts. Maximum values of flow acceleration parameter K lied in the range 1.3 ... 2.75∙10−6. Experimentally measured value was normalized to the developed turbulent boundary layer heat transfer coefficient and reached St/St Rex = 0.5–0.7 and indicated partial laminarization phenomenon. It was shown that the temperature recovery factor hasn't been affected by the flow acceleration. The results are compared to numerical heat transfer predictions using sst, γ-sst and γ-Re θ -sst turbulence models. Numerical calculations and experimental data were found to be in reasonably good agreement for low acceleration level with K less than 0.75∙10−6. Nevertheless, γ-sst turbulence model made slightly conservative estimation whereas in contrast γ-Re θ -sst model overestimates quantities for the moderate flow acceleration conditions. • Heat transfer in a small-size slot supersonic nozzle has been investigated. • Effect of flow acceleration on partial laminarization was assessed. • Temperature recovery factor does not depend on the flow acceleration. • Heat transfer coefficient reduces up to two times. [ABSTRACT FROM AUTHOR]

In this paper, some novel structures aimed to optimize the airside performance of H-type finned tube heat exchangers, such as longitudinal vortex generators, dimples/protrusions and grooves, are examined numerically. The effects of geometric parameters on the heat transfer, flow resistance and comprehensive thermal–hydraulic performances are investigated by means of Taguchi method. The optimum configuration of compound designs is determined on the basis of the optimizations of these structures. According to the results, for longitudinal vortex generators, dimples/protrusions and grooves, the heat transfer characteristics are significantly affected by the winglet type, vertical distance and groove diameter while the attack angle, dimple depth and groove type have prominent impacts on the flow resistance characteristics. The winglet type, vertical distance and groove number make the most contributions to the comprehensive thermal–hydraulic performance, respectively. In compound designs, the longitudinal vortex generator plays the most important role in the performance of heat exchangers. The comprehensive thermal–hydraulic characteristic of the optimal compound design exerts the improvements of 0.9–23.8%, 24.5–57.1% and 8.3–37.5% than those of LVGs, dimples/protrusions and grooves, respectively, when Reynolds number ranges from 4650 to 28,300. The enhancement of 4.5–16.6% can be achieved compared with the case built based on references. • The optimal parameters of longitudinal vortex generators, dimples/protrusions and grooves on H-type fins are determined by Taguchi method. • The innovative compound structures are designed based on the optimizations of each novel structure. • Longitudinal vortex generators have the primary impact on comprehensive thermal–hydraulic performances among three structures. • The optimal compound design can achieve significant improvements compared to single structure. [ABSTRACT FROM AUTHOR]

Jet impingement cooling is usually used in zones with extremely high thermal loads. The major factor that limits impingement heat transfer uniformity is the crossflow. The impingement cooling with return hole is applied to reduce the negative effect of crossflow on the target surface heat transfer in present paper. The influences of jet-to-plate spacing (H / D), impingement target surface curvature (α), temperature ratio (TR) and Reynolds number on flow field and heat transfer performance are investigated in detail. The results indicate that the correlation between the overall area average Nusselt number of target surface and H / D , the overall area average Nusselt number decreases by approximately 5.76% with H / D increases by one unit. Increasing the impingement target surface curvature is beneficial to expand the spanwise heat transfer. The overall area average Nusselt number increases by 295.5% for Reynolds number varying from 10,000 to 50,000 at TR = 0.910, and the overall area average Nusselt number increases by 8.68% for the TR varying from 0.668 to 0.910 at Re = 30,000. The results indicate that impingement cooling with return hole can achieve high heat transfer coefficient and uniform heat transfer distribution. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the flow-induced pitching, heaving or surging of an airfoil in a rectangular channel for heat transfer enhancement. The flow dynamics, airfoil fluttering response, and heat transfer performance are analyzed and discussed at four different Reynolds numbers (Re = 700, 1000, 1300 and 1500), three rotational axis locations (a x / C =0.25, 0.33 and 0.42), two airfoil perturbation modes (passive vs. active), and three combinations of multiple airfoil freedoms (pitch only, pitch and heave, and pitch and surge) with various stiffness coefficients. It is found that after Reynolds number reaches 1000, the pitch only airfoil starts the similar self-fluttering pattern, and the fluttering effect becomes the dominant influencer for increasing the Nusselt number compared to the Reynolds number effect. Placing the rotational axis at a x / C =0.42 achieves the highest thermal enhancement factor η of 1.4, while when a x / C =0.25, the airfoil oscillation diminish effect is vast because of its aerodynamic center. The passive pitching airfoil can achieve more than 6% of the thermal enhancement factor than the active ones. Adding another heave or surge degree of freedom to the pitching airfoil, the heat transfer enhancement of the channel can also be improved. The present work can facilitate future optimizations of airfoil-based flow-structure interaction system for heat exchanger channels. [ABSTRACT FROM AUTHOR]

• Thermal and hydrodynamic characteristics of staggered drop-shaped tubes were studied. • The effect of the number of tube rows N R on the performance was investigated. • The effect of increasing Reynolds number Re on the performance was analyzed. • The investigation covered 1.78 × 103 ≤ Re ≤ 18.72 × 103 and 1 ≤ N R ≤ 20. • New correlations for N u ¯ , f , and ε considering N R were developed. In this paper, for the first time, air-side correlations were developed and introduced to describe the average Nusselt number N u ¯ , friction factor f , and thermal − hydraulic performance ε of compact heat exchanger employing drop-shaped tubes considering the number of tube rows (N R). Drop-shaped tubes were arranged in a staggered formation and placed in 1 to 20 consecutive rows in the flow direction, each consisting of 7 tubes in the transverse direction. The Reynolds number (Re) varied from 1.78 × 103 to 18.72 × 103. The calculations were carried out using Ansys Fluent software package. Tube row and Reynolds number effects on the thermal and hydrodynamic characteristics of the studied heat exchangers were analyzed. The simulation results were in a good agreement with the available literature. The results showed that N u ¯ and ε increase, while f decreases as Re increases. Moreover, an increase in N R leads to an increase of f and a decrease of ε. The maximum values of N u ¯ were obtained for N R = 2, which were higher by 3.95–6.69% and 19.91–24.62% than those for N R = 1, and N R = 20, respectively. Heat transfer was stabilized only from the 19-row onwards. In addition, a 1-row heat exchanger had better thermal − hydraulic performance compared to that with a larger N R (ε for N R = 1 was about 22.32–27.24 times higher than those for N R = 20). The maximum deviation between the numerical results and the obtained correlations developed from them was ± 8.87%, which provides a useful reference for future studies and designs of heat exchangers with different row numbers of drop-shaped tubes. [ABSTRACT FROM AUTHOR]

This paper compares the thermohydraulic performance of varying curvature wavy microchannels, namely elliptical-wavy and sinusoidal-wavy microchannels. The amplitude of microchannels is fixed at 0.6 mm, and wavelengths are varied from 3 mm to 8 mm. Simulations are performed for the laminar flow of water (160 ≤ Re ≤ 900) considering three-dimensional conjugate heat transfer using ANSYS Fluent version 2020R2. The computational model has been validated by performing experiments on a serpentine-wavy microchannel, a special case of the elliptical-wavy microchannel, using micro-particle image velocimetry (μ-PIV) test facility. Both Nusselt number and friction factor increase with the increase in waviness. Elliptical-wavy microchannels have superior thermohydraulic performance than sinusoidal-wavy microchannels except for the channel of highest waviness at Re = 160. A microchannel with lower entropy generation has a higher thermohydraulic performance. Further, we observe centrifugal instabilities in both sinusoidal-wavy and elliptical-wavy microchannels at higher Reynolds numbers. Centrifugal instabilities in the elliptical-wavy differ from those in the sinusoidal-wavy microchannel by the asymmetric formation of additional vortices about the horizontal midplane in the channel cross-section. [ABSTRACT FROM AUTHOR]

This paper explores the effect of curved shape fin heights on open microchannel heat sink performance. Different geometries of rectangular, plano-convex, and plano-concave fins have been investigated by employing three-dimensional simulation. A variety of parameters including fin height, curve range coefficient, Reynolds number, and heat flux have been examined. At the same heat flux, Reynolds number, and fin height, the Nusselt number of the rectangular fin is lower than the two shapes of plano-convex and plano-concave. In Comparison of the two curved shapes, due to their geometries and fluid flow behavior, in the lower fin heights, the Nusselt number of the plano-concave is greater than the plano-convex. In the higher fin heights, the Nusselt number of the plano-convex is greater than the plano-concave. Even though at higher fins, where the Nusselt number of the plano-convex fins is greater than the rectangular and plano-concave shape, the plano-convex pressure drop is less than these two shapes. In a heat flux of 450 k W / m 2 , Reynolds number of 600, and a fins equivalent height of 0.6 mm, that the volume and mass of the fins are the same, there is an 18% difference between the maximum and minimum Nusselt number of plano-convex and plano-concave, by different curve coefficients. The obtained results are beneficial in designing novel shape fins in enhanced open microchannel heat sinks. [ABSTRACT FROM AUTHOR]

Mini-channel heat exchangers are extensively used to achieve heat dissipation with high heat flux, while the problem of flow maldistribution seriously impairs the thermal efficiency and operational reliability of heat exchangers. In this paper, the flow distribution characteristics of mini-channel heat exchangers with the conventional manifold model are numerically studied, and the influence of flow maldistribution on heat transfer is analyzed. To suppress the problem of flow maldistribution, an innovative approach for improving fluid uniformity by the built-in spiral baffle in the inlet manifold is proposed, and the effects of spiral baffles with different pitches and radii on flow distribution are discussed. Besides, the heat transfer characteristics and pressure drop under different manifold models are also compared. Finally, the variable-pitch spiral baffle is introduced to realize the stepped regulation of flow distribution. The results show that the conventional manifold model has serious flow maldistribution and causes the deterioration of heat transfer in front-end channels, especially at high Reynolds numbers. The improved manifold model greatly improves the uniformity of the fluid and temperature distribution inside mini-channel heat exchangers on the premise of sacrificing a small amount of pressure drop. The optimal pitch and radius of the spiral baffle are 20 mm and 9 mm respectively, and the flow maldistribution in almost all channels is controlled within the range of ±15% at this case. In addition, the variable-pitch spiral baffle further improves the flow uniformity of the mini-channel heat exchanger, and shows better overall performance than the equal-pitch spiral baffle. • The influence of flow maldistribution of the conventional manifold is analyzed. • An innovative method for improving the fluid uniformity is proposed. • The performances of spiral baffles with different parameters are compared. • Variable-pitch spiral baffle shows better comprehensive performance. [ABSTRACT FROM AUTHOR]

Swirling flows have applications in several areas which includes enhanced heat transfer and combustion. In these applications, swirling jets typically impinge on a target surface either orthogonally or at an angle. This paper presents an experimental and numerical investigation on the effects of jet-to-target spacing (z/d – 0.5 to 4) on heat transfer characteristics of orthogonally impinging swirling jets in comparison to geometrically similar cylindrical jet. Further, obliquely impinging jets (at z/d = 4) with anti-clockwise swirl (when viewing target surface from the jet) have been studied. Transient liquid crystal thermography experiments were carried out for above configurations at jet Reynolds number of 18,000 for orthogonal impingement and for Re ranging from 9000 to 18,000 for obliquely impinging swirling jet. The experimentally obtained local heat transfer when studied in conjunction with the flow-field predictions reveal that swirling jets can result in high heat transfer compared to conventional cylindrical jets at low z/d, where the area coverage of swirling jet is smaller than cylindrical jet. With increasing z/d, swirl jet impingement heat transfer showed significant reduction (while increasing coverage) while cylindrical jet impingement primary heat transfer zone was not very sensitive towards it. This is attributed to increased expansion of swirling jet exiting the nozzle at higher z/d, while cylindrical jet retaining its potential core for the z/d range investigated here. Obliquely impinging swirling jet had reduced stagnation zones due asymmetric arrangement of swirl passages with respect to target surface. [ABSTRACT FROM AUTHOR]

The main aim of this paper is to investigate the effect of nozzle geometry on the flow and heat transfer from a pulsed jet into a concave surface. Experiments have been carried out for pulsed circular jet and numerical simulations were done for circular, elliptical, rectangular and square geometries. Numerical solution was performed for the frequency ranges of 25 H z to 100 H z , the jet Reynolds number of 7000 , the dimensionless nozzle distance to concave surfaces of 2 and 5. Numerical results show a consistent agreement with experimental results and previous works. Accordingly, the geometry of nozzle directly affects the air entrainment ratio. In addition, with the increase of aspect ratio of the nozzle, average Nusselt number (Nu ave) decreases. By pulsating the inlet jet with pulse frequency of 100 H z , the average Nu number for circular/square and elliptical/rectangular jets increases 22 % and 15 % , respectively. Generally, the pulsation causes a reduction and an increase of the Nu ave at low and high frequencies, respectively, compared to that of the steady jet. At low frequencies, the Nu ave of circular jet decreases significantly in comparison to that of the square and elliptical jets. However, at high frequencies, circular jet shows higher Nu ave compared to other ones. As the distance between the jet and the concave surface increases, the effect of the nozzle shape on Nu ave decreases. [ABSTRACT FROM AUTHOR]

• Effect of novel positioned dual splitter plates on heat transfer is investigated. • Wake topology and its effect on heat transfer for dual splitter plates is discussed. • A mixed effect of the dual plates on the fluid forces around square cylinder is observed. • Substantial enhancement of 46% on the overall heat transfer is noticed. • Novel combined thermo-fluid regime is defined for the square cylinder with dual plates. In various applications of flow over a cylinder, the convective heat transfer is required to be enhanced for better operation by maintaining the operating temperature. In this paper, the influence of splitter plates on the fluid forces and heat transfer around a square cylinder is discussed thoroughly. The placement of the splitter plates is novel in its kind as these are placed at the top and bottom surfaces of a square cylinder instead of the conventional position i.e. downstream or upstream of the cylinder. Effect of this novel position of the splitter plates on the heat transfer along with the wake topology for a range of Reynolds number (Re) (60 ≤ Re ≤ 150) is not available in the previous study and accordingly, a rigorous investigation of the flow physics is presented here. Two splitter plates are utilized of the same geometry with varying lengths, locations, and orientations. It is found that the primary recirculation length decreases with increasing Re by introducing the splitter plates compared to a bare cylinder and the corresponding primary separation angle, ψ S1 is significantly affected by the geometry of the splitter plates. The splitter plates have a mixed effect on the fluid forces and heat transfer rate for all the considered Re. A maximum percentage of reduction on the mean drag coefficient and R.M.S (root mean squared) lift coefficient is found as 9% and 20%, respectively. Whereas, a maximum percentage enhancement of 40.7% is obtained on the heat transfer rate by introducing the splitter plates. However, the splitter plate geometry and its location can be decided according to the requirement of the heat transfer rate based on the application and the corresponding demand on the fluid forces. [Display omitted] [ABSTRACT FROM AUTHOR]

• IR Thermography is applied to measure heat transfer from impinging swirling jets. • The swirl number and impingement distance influence the shape of the heat transfer distribution. • The Reynolds number affects essentially the magnitude of the heat transfer rates. • Four different regimes are identified as a function of the swirl degree and impingement distance. • Empirical correlation laws for the area-averaged Nusselt number in these regimes are derived. This paper reports on a parametric study of the impingement heat transfer from swirling jets issuing from a circular nozzle. The swirl velocity component is imparted to the flow via a tangential entry into a recirculation chamber located upstream of the nozzle; the degree of swirl is varied by changing the angle of the channels leading to the chamber with respect to the radial direction. The effects of the dimensionless impingement distance (H / D = 1 , 2, 3, 4, 6, 8, 10, 12, 14 with D being the nozzle diameter), the swirl number (S = 0 , 0.078 , 0.22 , 0.37 , 0.54) and the Reynolds number (Re = 20,800 , 31,100 , 41,500) on the spatial distribution of the heat transfer are investigated by means of infrared thermography and the heated thin foil sensor. The present results show that the swirl number and the impingement distance significantly influence the structure of the heat transfer distribution, while the Reynolds number affects essentially the magnitude. Four distinct ranges with different behaviours can be identified by distinguishing between weakly or non swirling jets (0 ≤ S ≤ 0.22) and moderately swirling jets (0.37 ≤ S ≤ 0.54) and between short and long impingement distances (1 ≤ H / D ≤ 4 and 6 ≤ H / D ≤ 14 respectively). A comparative assessment of the performance of the swirling jets is carried out by focusing on the area-averaged Nusselt number distributions. It is found that in the range of short impingement distances and over relatively small target areas an enhancement of the heat transfer rates can be obtained at a small extent by adding a weak swirl with no detrimental effect on the uniformity of the distribution and at a larger extent by a moderate swirl with a deterioration of the uniformity. For long impingement distances no enhancement is observed, although swirl yields a significant reduction of the non-uniformity. Finally, correlation laws of the area-averaged Nusselt number as a function of the control parameters are derived in the four regimes identified. [ABSTRACT FROM AUTHOR]

This paper proposed a swirl chamber with circumferential ribs to avoid the impacting of cross-flow to jets, thus enhancing the penetration ability of jet and swirl cooling heat transfer intensity. To reveal the mutual effects between jets and cross-flow, the heat transfer features and flow structures under different Reynolds numbers of smooth swirl chamber and novel rib-roughened swirl chambers were studied and compared using 3D steady numerical analysis. The influence of rib height ratio on flow and heat transfer features was investigated further. The investigation revealed that for smooth swirl chamber, the cross-flow seriously weakened the heat transfer intensity. In contrast, the novel rib-roughened swirl chamber had an excellent suppression effect on cross-flow, thus greatly enhancing the penetration ability of jet and strengthening the swirl cooling heat transfer intensity. Under the Reynolds number of 10,500, for smooth swirl chamber, the peak number of circumferentially-averaged Nusselt number decreased from 80 to 58 from the first pitch to the last pitch. While for the case that rib height ratio is equal to 0.5, it increased from 80 to 120. In addition, the suppression effects of circumferential ribs on cross-flow increased when the rib height ratio increased, likewise the heat transfer enhancement effect. When the rib height ratio is less than 0.5, the Nusselt number ratio was larger than the friction factor ratio, while when the rib height ratio is larger than 0.5, the Nusselt number ratio was less than the friction factor ratio. The best thermal performance can be achieved when the rib height ratio was 0.5. Besides, the heat transfer enhancement effect of circumferential ribs was more significant under a higher Reynolds number in the studying range. [ABSTRACT FROM AUTHOR]

The secondary flow has a strong promotion effect on heat transfer in the tube. In order to further utilize the secondary flow phenomenon to strengthen heat transfer of the helical tube, in this paper, a secondary flow enhanced heat transfer helical tube is developed, and stable secondary flow with high strength is obtained. The structure parameters are optimized with the highest comprehensive evaluation factor as the goal. The results show that compared with helical transverse-groove tube and helical grooved tube, helical longitudinal-groove tube has the best heat transfer performance. The new structure's enhanced heat transfer effect for high- Pr number fluid is better than that of low- Pr number fluid, and the enhanced heat transfer effect is not affected by the inlet Reynolds number. The helical angle of helical groove has a greater influence on the comprehensive evaluation factor, while the groove depth and the number of heads have a smaller influence on the comprehensive evaluation factor. The optimal parameter scheme is: The groove depth is 0.5 mm, the helical angle of helical groove is 82°, the head number of helical groove is 1, the comprehensive evaluation factor of helical longitudinal-groove tube compared with ordinary helical tube is 1.173 • The secondary flow and heat transfer characteristics of helical longitudinal-groove tubes are investigated. • Slotting near the vertical direction of the cross-section has a greater impact on the formation of secondary flow. • The heat transfer enhancement effect of the high Pr number working fluid is better than the small Pr number working fluid. • The structural parameters are optimized by multi-factor orthogonal test and local parameter single-factor analysis method. [ABSTRACT FROM AUTHOR]

All studies regarding the double passages channel use a rigid baffle to separate the two passages. In the current paper, we use a flexible separator and inspected its role in enhancing the performance of the heat transfer. The cold fluid is forced upward in the channel which is heated isothermally by different temperatures on its walls. A flexible sheet is fixed from its two ends and equally separate the channel to two passages. The problem is investigated numerically using the finite element method coupled with the arbitrary Lagrangian-Eulerian approach. The problem is casted in dimensionless form and several pertained parameters have been inspected, like Reynolds number in each passage, elasticity of the separating sheet and the temperature ratio of the channel walls. Results are promising where it is found that the enhancement of heat transfer is possible passively in either passage with the aid of using a flexible separator rather than rigid separator. A 6% enhancement in the Nusselt number of the left passage, which comprises the higher temperature, is obtained by making the separator flexible. The flexible sheet is found to increase the pressure drop to 200% in some cases which leading to decline the thermal performance criterion to lower than unity. However, the thermal performance criterion TEC was feasible in the right passage which subjects to lower temperature. • A monolithic finite element approach using high-order schemes in space and time is utilized. • Heat transfer enhancement is possible passively in a passage with the aid flexible separator rather than rigid separator. • A 6% enhancement in the Nusselt number of the left passage is obtained by making the separator flexible. • The flexible sheet increases the pressure drop to 200% in some cases leading to decline the thermal performance criterion. • Thermal performance criterion TEC was feasible in the right passage which subjects to lower temperature. [ABSTRACT FROM AUTHOR]

• A bionic heat sink with micro barchan-dune-shaped humps is proposed. • The micro dune can reinitialize the boundary layer by inducing a secondary flow. • The micro dune generally outperforms traditional fins in overall heat transfer performance. This paper proposes an innovative bionic heat sink with micro barchan-dune-shaped humps for improving the flow and heat transfer performance of the smooth narrow channel. The effects of the Reynolds number of the main flow (Re) and pitch distance of the dune array (P) on the thermal-hydraulic performance are investigated. In this work, the air is the working flow. The Reynolds number range is considered to cover the regimes of laminar, transitional, and turbulent, Re = 860–6,050, and the pitch distance (P) equals 8, 12, and 16 mm. The results show that the micro barchan-dune-shaped hump can induce a secondary flow downstream of the slip face and enhance the local heat transfer performance. For the multi-dune models, the case with P = 16 mm provides the highest overall thermal performance parameter (η). The η value decreases monotonically with decreasing P in the considered range. In addition, compared with other micro pin fin shapes (including the hexagonal cylinder, square cylinder, and triangular cylinder), the η of the barchan dune shape is almost identical to that of the hexagonal cylinder and exceeds the other two shapes by more than 20%. [Display omitted] [ABSTRACT FROM AUTHOR]

• Flow and heat transfer characteristics of jet array impingement with effusion holes are investigated. • A total of 300 simulations are tested for five hole arrangements. • The radial acceleration, turbulence, and jet potential core strongly affect impingement heat transfer. • Predictions of previous Nusselt number correlations for jet array impingement are evaluated. • A new Nusselt number correlation applicable to five hole arrangements is proposed. Impingement/effusion cooling systems have been studied extensively over the past few decades to constantly improve cooling performance as the turbine inlet temperature of modern gas turbine engines has increased. However, a Nusselt number correlation is less developed for an impingement/effusion cooling system with effusion holes in its impingement plate than for jet array impingement without effusion holes. Moreover, Nusselt number correlations, available for a wide range of geometric and flow parameters with various hole arrangements, are sparse for gas-turbine casing cooling systems. Therefore, this study investigated the effects of these parameters on the flow and heat transfer characteristics of an impingement/effusion cooling system that uses a jet array of effusion holes to cool a gas-turbine casing. The circular holes in the parallel jet and impingement plates were arranged in five different hole arrangements. Air forced through holes in the jet plate struck the impingement plate then discharged through the effusion holes of the impingement plate. Three-dimensional simulations were performed to examine the effects of hole pitch, distance between the jet and impingement plates, ratio of total jet area to heat transfer area, and Reynolds number based on the jet hole diameter on impingement heat transfer. A total of 300 simulations were run for the five hole arrangements to obtain the local and average Nusselt numbers on the impingement plate. Our results show that the radial acceleration and turbulence near the impingement plate, and the length of the jet potential core strongly affect the impingement heat transfer. This study also evaluated previous Nusselt number correlations for jet array impingement. This paper presents a new correlation to predict the average Nusselt number of jet array impingement applicable to the five hole arrangements. [ABSTRACT FROM AUTHOR]

As an effective technology to reduce flow resistance, drilling holes on the surface of vortex generators had been studied extensively. In this paper, thermal performance of punched rectangular winglet vortex generators (PRWVGs) in turbulent regime (Reynolds number was in the range of 9,090 ∼ 21,210) was studied experimentally and numerically. Air was applied as the working fluid. The PRWVGs were inserted into the tube with three attack angles (α = 30 ° , 45 ° , 60 °). The punched holes were set in three different positions (g = 3 m m , 5 m m , 7 m m) in the vertical direction and at three different hole heights (d = 1 m m , 3 m m , 6 m m). The performance of PRWVGs was quantified by the heat transfer coefficient ratio (h / h 0), the friction factor ratio (f / f 0), and a combined thermal enhancement factor (T E F) of the two. The results revealed that h / h 0 as well as f / f 0 values of PRWVGs compared with those of planar VGs (without holes) were reduced. The flow and temperature fields of different cases were obtained by numerical simulations, and the experimental results were verified by analyzing the fluid flow behaviors. The results showed that the jet flow (defined as a jet emanating from the punched hole of a VG), would enhance the heat transfer efficiency in the recirculation area. And the T E F value of VGs was improved by punching holes on the surface of VGs. However, the shape of the jet flow depended on the position of the hole. When the position of the hole was lower in the vertical direction, the jet flow was effective in increasing its T E F value. Additionally, the size of the hole had a large effect on the T E F value. Besides, entropy generation analysis showed that PRWVGs were thermodynamically advantageous. When d = 1 m m , w = 5 m m , the Bejan number reached the minimum. Under this condition, the proposed PRWVGs performed best in practice. When the height of the hole was d = 1 m m , T E F was the maximum, which was 1.25 at α = 45 ° and R e = 9,090. [ABSTRACT FROM AUTHOR]

· Behavior of local heat transfer coefficient in plane diverging channels is described. • Local Stanton number should be based on flow parameters at diverging channel inlet. • Empirical equations for local Stanton number are based on Re and Kays parameter. • Kinematic structure and TKE in plane diverging channels are estimated experimentally. • Correlation between local heat transfer and local Reynolds stresses is revealed. Heat transfer in diverging channels is essentially different from the one in channels of constant cross section. The mechanism of heat transfer enhancement in diverging channels compared to the channels of constant cross section is based on increased turbulence intensity. The present paper suggests new approach to the analysis and generalization of heat transfer data. Heat transfer on the wall of a diverging channel should be considered similarly to heat transfer from a plate instead of heat transfer in a channel. The data on heat transfer coefficient on the wall of a plane diverging channel at different expansion angles can be generalized by the Stanton number as a function of the Reynolds number and Kays acceleration parameter, St∼ f (Re, K). This function has characteristic sections along the wall that are typical of laminar, transitional and turbulent boundary layers. Stanton and Reynolds numbers based on the distance from the inlet and the velocity at the diverging channel inlet can be employed for data generalization in all the considered ranges of geometries and flow regimes. Kinematic structure of flow is examined to reveal hydrodynamic mechanisms behind heat transfer formation. Optical measurements provide the profiles of velocities and turbulence parameters in characteristic sections of the channel. Mechanism of turbulent structure formation in diverging channels is examined. Correlation between hydrodynamics and heat transfer is analyzed. Correlation between local coefficients of heat transfer and near-wall Reynolds stress is revealed. [ABSTRACT FROM AUTHOR]

• Self-excited rotational squares enhance heat and mass transfer in a microchannel. • Transport details are revealed under all 5 distinct rotation modes with 5 < Re < 300. • Both local and overall enhancement efficiencies are quantitatively determined. • Phase diagrams of enhancement index are drawn relative to Re and axial position. • Underlying mechanisms are analyzed based on cylinder dynamics and flow structures. The flow disturbance technique is an effective method to enhance the heat and mass transfer efficiency in microchannels. This paper numerically investigates the influence of two side-by-side freely rotatable square cylinders on microchannel flow, heat transfer, and mass transfer under a Reynolds number (Re) range of 5‒300. Results indicate that the cylinders show five different vortex-induced rotation modes, namely the rotation mode (Re < ∼10), oscillation mode (∼10 < Re < ∼70), stationary mode (∼70 < Re < ∼90), random mode (∼90 < Re < ∼250), and reversal rotation mode (Re > ∼250). The vortices generated by rotations of the cylinders disturb the flow field significantly, thus promoting heat and mass transfer in the microchannel. As Re increases, the average Nusselt number (Nu) of the channel first rises slowly then sharply, specifically, from 3.50 when Re = 5 to 8.35 when Re = 75, and then to 42.45 when Re = 300. Under a small Re , the rotating cylinders only affect the local heat transfer near them, while only when Re > 100 can the cylinders significantly promote the heat transfer in a relatively long distance in the microchannel. This behavior should be attributed to the insufficient vortex disturbance under small Reynolds numbers. Mass transfer is scrutinized by releasing fluorescein at the upper half of the channel entrance, and mixing efficiency (η) is introduced to characterize the mixing situation. We show that when Re = 100, the substances in the channel are almost completely mixed (η = 0.98) compared with η = 0.84 for stationary cylinders; beside, as Re increases, the position where the mixture reaches a steady-state gradually moves forward in the channel, and when R > 100, the distribution of η along the channel seldom changes with Re. Considering fluid pressure loss, heat transfer enhancement index φ and mass transfer enhancement index ϕ are introduced to compare heat and mass transfer capability between microchannels set with rotatable cylinders and stationary cylinders, and phase diagrams relative to Re and x * are drawn. Analysis reveals that the freely rotatable cylinders better improve the heat and mass transfer with a maximum φ of 1.6 at Re ≈ 25, x * ≈ 12 or Re ≈ 100, and maximum ϕ of 1400 at Re ≈ 50, x * ≈ 10‒25. [ABSTRACT FROM AUTHOR]

The impingement cooling is used to protect turbine components under high thermal load. This paper conducts a numerical study of heat transfer characteristics of turbine leading edge with lamilloy cooling structure. Target surface curvature and jet-to-surface spacing under various crossflow Reynolds number conditions are considered. Meanwhile, the target surface is dimpled to enhance heat transfer capability. Besides, staggered film holes are considered in the computational domain. The results show that the crossflow induces a counter rotating vortex pair that greatly increase the near wall turbulence and local heat transfer. The vortex pair forms the bound for a U-shaped high heat transfer region caused by the jet impingement. The target surface curvature tends to enlarge the relative area of the U-shaped region. The increased crossflow strength weakens the jet cooling effect, the total heat transfer performance is therefore decreased. Large jet-to-surface spacing enhances the impingement cooling when crossflow is weak but the total heat transfer will not recover as crossflow Reynolds number get higher compared with small and medium spacing cases. • Computational fluid dynamics (CFD) models used to simulate turbine leading edge internal heat transfer. • Impingement cooling configured with dimpled surface and film holes. • Compound influence of jet-to-surface spacing, surface curvature under crossflow. [ABSTRACT FROM AUTHOR]

Taylor–Couette–Poiseuille flow commonly occurs in rotating machinery in the annulus between the outer casing and rotating shaft as part of lubrication or thermal management systems. To improve efficiency of these machines, industry is beginning to work with low viscosity, high density fluids operating at high pressures, such as supercritical carbon dioxide turbines. Within existing literature, vast high quality heat transfer data exists for air under Taylor–Couette–Poiseuille flow regimes. However, for other fluids, these data are limited in their experimental coverage, especially at high Taylor numbers (i.e. > 1 × 10 10). This paper presents first of its kind experimental data for supercritical carbon dioxide. The data are analysed to provide insight into the heat transfer processes and to provide a Taylor–Couette–Poiseuille flow heat transfer correlation. Operating conditions of the experimental campaign cover pressures of 7 and 10 MPa, a temperature range of 315 to 333 K, and rotational speeds of 10 to 25 kRPM. This equates to Taylor number and axial Reynolds number ranges of 9.52 × 10 10 to 8.73 × 10 11 and 3.41 × 10 4 to 5.4 × 10 4 , respectively. The range of conditions tested covers the operational envelope of those found in thermal management systems for supercritical carbon dioxide turbine designs. It is found that heat transfer of supercritical carbon dioxide is significantly higher than that of ideal gases (e.g. air) for the same Taylor number range. Further findings show that change in fluid properties near the critical point (i.e. heat capacity and density) increase the Nusselt number from 1.5 × 10 3 at 10 MPa to 3.1 × 10 3 at 7 MPa at an effective Reynolds number of 2.5 × 10 6. A dimensional analysis shows the importance of capturing the change in fluid properties when near the critical point and to account for tangential energy transport. To capture this we introduce a non-dimensional parameter, Ψ , being the ratio of tangential energy flux to inertial energy flux. Including Ψ in heat transfer correlations improves their validity by capturing additional mechanisms that influence heat transfer. The suitability of correlations including Ψ are demonstrated using air and supercritical carbon dioxide data sets. Finally, analysing heat transfer trends across the range of Ψ tested provides insight on how heat transfer mechanisms vary with operating condition. This work provides a vital step forward for the unification of Taylor–Couette–Poiseuille flow heat transfer correlations across multiple experimental data sets. [ABSTRACT FROM AUTHOR]

Impingement cooling is an important turbine blades cooling technique, but it is often not as efficient as required due to the deflection of downstream jets caused by crossflow. In this paper, a novel corrugated target surface is proposed to convert downstream oblique impingement into orthogonal impingement for higher heat transfer. To prove this design, the flow and heat transfer characteristics of impingement cooling with conventional flat target surface (Baseline case), full-corrugated (FC) target surface, and semi-corrugated (SC) target surface are compared over the jet Reynolds number ranging from 15,000 to 45,000. Results show that the Nusselt number of full-corrugated target surface is almost the same as that of flat target surface at the region corresponding to the first five jets, but its peak value and uniformity are significantly improved at the region corresponding to the last four jets, especially under high jet Reynolds numbers. In addition, with the increase of corrugation depth (H), the total heat transfer capacity and area-averaged Nusselt number increase, but this comes at the cost of increased friction loss. Comprehensively evaluating heat transfer and friction loss, only the FC with H = 0.8 D case always has better thermal performance than Baseline case under all computed conditions, and the maximum improvement can reach 5.8%. Last but not least, the SC with H = 0.8 D case is proposed based on the FC with H = 0.8 D case to further reduce the friction loss, and results show that its thermal performance is improved significantly due to the decreased friction loss and almost unchanged heat transfer. [ABSTRACT FROM AUTHOR]

The heat transfer performances of pipes with twisted tape inserts having periodically spherical dimple-protrusion patterned walls are numerically investigated by Ansys Fluent under a constant heat flux of 5 W/cm2. The inserts having three different densities (N = 0, 30, and 45) and three different kinds of arrangement (composed of only dimples, only protrusions, and a combination of dimples and protrusions) with a constant twist ratio of 3 were tested for Reynolds numbers (Re) within the range from 3000 to 27000. Comparing the turbulence models proposed the validated realizable k-ϵ model coupled with enhanced wall function to employ on the numerical simulations. While the highest Nusselt number (Nu) and friction factor (f) values for the presented study were found for the dense protruded twisted tape (D-PT) arrangement at Re values lower than 18000 and for dense dimpled-protruded twisted tape (D-DPT) arrangement for the Re greater than 18000, dimpled-protruded twisted tape (DPT) and dimpled twisted tape (DT) sequences reached the lowest f values of 0.365 and 0.322 at Re = 3000, respectively. D-DPT and D-PT sequences reached the highest thermal performance factor (TPF) values of 1.508 and 1.478 at Re = 3000, respectively. All patterned twisted tapes performed better Nu than conventional twisted tapes in the analyzing range and performed better TPF than typically twisted tapes in the Re region of less than 6000. [Display omitted] • The effect of DT, PT, DPT, D-DT, D-PT, and D-DPT on heat transfer enhancement performance was numerically investigated. • All patterned twisted tapes performed better TFT than typically twisted tapes in the Re region of less than 6000. • D-DPT and D-PT sequences reached the highest TPF values of 1.508 and 1.478 at Re = 3000, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the large eddy simulation of turbulence in coaxial cylinders determines the influences of slit structure on flow characteristics and heat transfer behavior. The cross-sections of slits are trapezoid, rectangle and ellipse respectively. The field synergy principle and entropy generation are utilized to comment on the heat transfer properties. The results demonstrate that slit shape influences the heat transfer capacity remarkably. The trapezoid and rectangle slit models show backflow in the slit region, while ellipse slit model has no backflow, thus being expected to result in the fluid exchange is more sufficient. In terms of field synergy principle, the ellipse slit model possesses the smallest field synergy angle under different Reynolds numbers which reveals the finest collaboration relationship of velocity with the temperature fields. The thermodynamic analysis indicates that thermal entropy generation has been promoted as a major contributor to total entropy generation, and the entropy production value of the ellipse slit model is small, leading to the energy utilization rate is better than other models. The finest heat transfer efficiency is found with the ellipse slit model for given Reynolds number range. [ABSTRACT FROM AUTHOR]